Cable television systems which provide pay televisions service to subscribers via a cable connection for a fee are well known in the United States and elsewhere. Since cable signals may be connected to receivers of both subscribers and non-subscribers (e.g., former subscribers who have terminated service but remain connected to the cable), the cable signals typically are scrambled to prevent unauthorized use by non-subscribers. Scrambling is often accomplished simply and economically by injecting an interferer signal into the video signal so as to render the video signal unviewable. Unscrambling at a subscriber's receiver is accomplished by using a notch filter to trap out the interferer signal.
In accordance with the principles of the present invention, it is herein recognized that the effectiveness of cable television scrambling/descrambling systems of the interferer signal type may be jeopardized by receivers of non-subscribers which incorporate a video signal equalizer to compensate for phase or frequency disturbances caused by transmission channel (cable) characteristics. Such equalizers typically operate to make certain characteristics of a received video signal, e.g., frequency, phase or energy, match or approximate a desired response or contour.
A television signal deghosting system is a type of video signal equalizer which removes or reduces video signal multipath distortion. These systems can be used with both broadcast and cable television signals, and may employ techniques at a receiver for comparing a received test signal with an ideal version of the test signal in order to configure an adaptive filter to remove multipath ("ghost") components from the received signal. One deghoster system, described in U.S. Pat. No. 5,065,242--Dieterich et al., incorporates a repetitive pseudorandom number sequence test signal in a horizontal line within a vertical blanking interval of a television signal. At a receiver, the received test signal and a locally provided ideal version of the test signal are compared and processed for generating coefficients for an adaptive multipath correction filter.